1. Field of the Invention
This invention relates to an optical recording medium, having a groove formed along a recording track, a master disc used for the preparation of the optical recording medium, and to an optical recording and/or reproducing apparatus for recording and/or reproducing information signals for an optical recording medium prepared using this master disc.
2. Description of Related Art
As an optical recording medium, an optical disc for optically recording and/or reproducing information signals is being practically used. Among such optical recording mediums, there are a replay-only optical disc having embossed pits corresponding to data, previously formed on a disc substrate, a magneto-optical disc for recording data by exploiting photomagnetic effects, and a phase-change optical disc for recording data by exploiting phase changes in a recording film.
Of these optical discs, those which permit writing, such as magneto-optical disc or phase-change optical disc, are usually formed with grooves extending along a recording track. By the groove herein is meant a so-called guide groove formed for extending along the recording track mainly in order to permit tracking servo. An area between neighboring grooves is termed a land.
In an optical disc, carrying these grooves, tracking servo is performed based on push-pull signals obtained from light reflected and diffracted by the grooves. The push-pull signals are obtained by detecting the light reflected and diffracted by the groove by two photodetectors, arranged symmetrically with respect to the track center, and by taking the difference between the outputs of the two photodetectors.
In these optical discs, a high recording density was achieved by improving the playback resolution of an optical pickup loaded on a reproducing apparatus. For improving the playback resolution of an optical pickup, a shorter wavelength λ of the laser light is used for data playback or a larger numerical aperture NA of an optical lens is used for converging the laser light on the optical disc.
The wavelength λ of the laser light used for data playback, the numerical aperture NA of the optical lens and the values of the track pitch are shown for CD, MD, MDData2, DVD-RW and DVD-ROM are shown in Table 1. It is noted that CD, MD, MDData2, DVD-RW and DVD-ROM are all trademarks for different types of the optical discs.
TABLE 1laser wavelength [nm]NAtrack pitch [nm]CD and MD7800.451600 DVD-ROM6500.60740DVD-RW6500.60800MD Data26500.52950
In a conventional optical disc, in order to achieve a high recording density, a narrow track width was realized by shortening the wavelength λ of the laser light or by enlarging the numerical aperture NA of the optical lens, as shown in Table 1.
Meanwhile, in the conventional optical disc, the track pitch is on the order of ½ to ⅔ of the cut-off frequency of the optical pickup of the reproducing apparatus. The cut-off frequency herein means a frequency for which the amplitude of the playback signal is approximately 0. It is noted that the cut-off frequency is represented by 2NA/λ, where λ is the wavelength of the laser light used for data reproduction and NA is the numerical aperture of an optical lens used for converging the laser light on an optical disc.
The reason the track pitch is on the order of ½ to ⅔ of the cut-off frequency is that, for realizing stable tracking servo or track seek, it is necessary to achieve a sufficiently high level of a signal required for tracking servo or seek.
For example, in an optical disc in which information signals are of high density, push-pull signals are used as tracking error signals. If desired to achieve stable tracking servo, the amplitude ratio of push-pull signals needs to be on the order of 0.14 or higher. Also, cross-track signals are used for traverse counting in seeking and for detecting track radial positions. For stable seek, a cross track signal amplitude needs to be on the order of 0.06 or more. If, in a conventional optical disc, the amplitude ratio of the push-pull signal is to be 0.14 or more and the cross track signal amplitude is to be 0.06 or more, the track pitch has to be on the order of ½ to ⅔ of the cut-off frequency.
Meanwhile, a push-pull signal is obtained by detecting light reflected and diffracted by a groove by two photodetectors A, B arranged symmetrically with respect to the track center and by taking a difference (A−B) of outputs from the two photodetectors A and B, as shown in FIG. 1. The cross-track signals are obtained by taking the sum of outputs of these two photodetectors A and B.
The amplitude ratio of the push-pull signals is represented by C/Mmax, where C is the maximum amplitude of the push-pull signal, as shown in FIG. 2. The maximum amplitude ratio of cross-track signals is represented by D/Mmax, as shown in FIG. 2, where D is the maximum amplitude of cross-track signals and Mmax is the maximum value of a sum signal M of signals from the two photodetectors A and B, that is the value of the sum signal M on the mirror surface of the disc.
Meanwhile, in an optical recording medium, such as an optical disc, it is desired to raise the recording density of the recording signals further. To this end, it suffices to narrow the gap between neighboring grooves to narrow down the track pitch. However, if, in a conventional optical recording medium, the track pitch is too narrow, the signals necessary for tracking servo or seek cannot be obtained with a sufficient level, such that stable tracking servo or seek cannot be achieved.
For example, in MD Data2, the track pitch is 0.95 μm, with the push-pull signal amplitude ratio being on the order of 0.30. In such case, the push-pull signal amplitude ratio is large to permit stable tracking servo to be achieved. However, if, in a configuration similar to that of the MD Data2, the track pitch is 0.75 μm, the push-pull signal amplitude ratio is as low as approximately 0.07. This push-pull signal amplitude ratio is too low to realize stable tracking servo.
If, in a configuration similar to one of MD Data2, the track pitch is set to 0.75 μm, the push-pull signal amplitude ratio is as low as approximately 0.06, while the cross-track amplitude ratio is as low as approximately 0.05. These values of the push-pull signal amplitude ratio and the cross-track amplitude ratio are to low to realize stable tracking servo or seek.
Thus, in a conventional optical recording medium, if the track pitch is too narrow, it becomes impossible to achieve signals required for tracking servo or seek with a sufficient level, such that stable tracking servo or seek cannot be achieved, with the result that, with the conventional optical recording medium, the recording density cannot be improved further.